The present invention relates to a surface electrode for transcutaneous electrical communication, and, in particular, to a surface electrode providing sustained mechanical and electrical performance over long-term use, especially for use underneath a cast on the limb of a patient.
Electrodes which are used to deliver electrical stimulation through the surface of the skin generally require the use of a conductive liquid or solid gel, often termed “hydrogel”, to provide a continuous conductive path between the skin and the current source. Conductive gels contain a salt (typically KCl or NaCl) in order to achieve the requisite electrical current flow. The preferred gel is one with a high salt content, since such a gel produces a better conductor than that obtained when using a gel with a low salt content. In addition, the use of a high salt content gel typically requires less skin abrasion at the time of application to reduce the impedance of the skin-electrode interface after subsequent electrode application.
For ease of use, it is often desirable to apply the conductive liquid or solid gel at the point of manufacture, creating a “pre-gelled” electrode. U.S. Pat. No. 4,559,950 to Vaughn and U.S. Pat. No. 5,309,909 to Gadsby describe such electrodes. Pre-gelled electrodes save the step of manually applying the gel to the electrode at the time of electrode application and speed the application process considerably.
Known gels are typically hydrophilic, to improve conductivity of the gel, and perhaps more importantly, to slow the gradual dehydration of stored, sealed electrodes. It is reported by United States Patent Application No. 20020117408 to Solosko, et al., that the shelf life of an electrode pad is largely determined by the length of time it takes for enough water moisture to evaporate out of the hydrogel and escape the package of the pad. It is further articulated that as moisture escapes from the packaging, the electrical properties of the electrode pads become increasingly compromised.
This problem is a critical one for numerous and varied medical applications. For example, when electrode pads are utilized with a defibrillator, a very significant factor includes changes in small and large signal impedance values between a patient and a defibrillator. As the hydrogel dries out, the impedance values increase, making it more difficult to monitor electrical signals from the patient, obtain transthoracic impedance, and deliver energy into the body.
Water loss can affect the mechanical properties of the hydrogel as well. In some hydrogels, the loss of water causes the hydrogel to skin over or solidify, especially around the edges, which inhibits the ability of the hydrogel to adhere to the skin. This partial or complete loss of adhesion can render an electrode useless since it cannot then create or maintain an effective contact with the patient's skin. Thus, water loss from the electrode pad can prevent or attenuate receipt of electrocardiogram (ECG) signals by a defibrillator. In addition, water loss from the electrode pad can alter the delivery of defibrillation energy from a defibrillator to the patient.
Additionally, poor or uneven contact of the electrode pad with the skin of a patient may unduly concentrate energy transfer during defibrillation into areas that exhibit good skin contact. Higher than usual current densities resulting from poor or uneven skin contact can cause skin burns. If the current is not delivered to a patient in the manner for which the electrode pad was designed, the resulting treatment delivered to the patient may be altered, compromising patient outcome.
Although highly hydrophilic hydrogels slow the gradual dehydration of stored, sealed electrodes, and also slow the gradual dehydration of electrodes on most exposed skin surfaces, the changes in mechanical and electrical properties over the long term exceed the tolerances in many medical applications. Moreover, highly hydrophilic gels have distinct disadvantages in applications requiring long-term, “wet” contact between electrode and skin, e.g., in a closed environment underneath a cast. In such wet environments, hydrophilic gels absorb water and/or sweat on the skin surface, causing swelling and even disintegration of the conductive pad.
U.S. Pat. No. 4,300,575 to Wilson discloses an air-permeable disposable electrode having a conductive silicone pad adapted to receive one end of an electrical lead from an active electrical instrument. The pad engages a permeable conductive element that is skin engageable through a permeable conductive adhesive coating. The permeable conductive element is formed primarily from karaya and carbon black so that the element can “breathe”. The permeable conductive adhesive coating is likewise formed primarily from karaya so that the coating can also “breathe”. A cover, also of air-permeable material, is provided with an adhesive on the inner side.
U.S. Pat. No. 4,300,575 states that it is essential that the conductive element and conductive adhesive coating provide an air-permeable covering to the skin of a patient so that the skin can “breathe” through the conductive element and adhesive coating. This enables the disposable electrode to be continuously used for relatively long periods of time on the order of a week or longer, whereas other disposable electrodes must be removed in a few days, at least from patients that have skin reactions resulting from contact of the skin with the disposable electrode.
As taught by U.S. Pat. No. 4,300,575, the permeable, conductive, adhesive coating interfacing with the skin of the patient electrode has no macroscopic exposed surface area (e.g., perforations), relying on microscopic, highly tortuous channels through which air molecules may permeate. Moreover, U.S. Pat. No. 4,300,575 fairly teaches away from a conductive interface layer having macroscopic exposed surface area, because such a configuration compromises the efficacy of the adhesive coating, and reduces the electrical contact area.
Moreover, the breatheability of the electrode is severely compromised by a conductive silicone pad, which serves to transfer electrical current from the electrical lead wire to the conductive element. The surface area of the silicone pad must be relatively large to convey the current. Typically, the silicone pad has a diameter of about 0.325 inches, and a thickness of about 0.020 inches. The skin lying underneath the relatively large area covered by the silicone pad is substantially sealed from air contact.
U.S. Pat. No. 4,367,755 to Bailey teaches an electrode for various stimulating applications, such as pain control. The electrode has a backing layer of conductive silicone rubber having a multiplicity of perforations. These perforations, preferably in the form of a rectangular grid pattern, are provided over the surface of the backing layer, with each perforation extending completely through the backing layer.
It should be appreciated that the perforations disclosed by U.S. Pat. No. 4,367,755 to Bailey are disposed solely in the backing layer. As illustrated and described, the conductive flexible pad interfacing with the skin, which is made of polymer gel or karaya gum, is continuous, and devoid of perforations. Like U.S. Pat. No. 4,300,575, U.S. Pat. No. 4,367,755 relies on microscopic, highly tortuous channels within the conductive flexible pad to enable the permeation of air molecules.
Although such a pad may be permeable to air molecules, the electrode taught by U.S. Pat. No. 4,367,755 to Bailey is, in many cases, inadequate for mass transport of sweat accumulating on the surface of the skin, through the pad, to the inside of the backing layer, and from the inside of the backing layer, via the perforations therein, to the atmosphere. The evaporation of water (in the sweat) is a function of the total amount of air diffusing from the environment to the skin surface, multiplied by the degree to which the air is unsaturated with respect to water vapor. Since the total flow of air reaching the skin surface is relatively small, the capacity to remove water at the skin surface is correspondingly low.
The thickness of the conductive, adhesive pad is a critical parameter in the delivery of air to the skin surface, and in the transport of water vapor from the skin surface out through the pad. With increasing thickness, the diffusion through the pad is reduced. The conductive, adhesive pad taught by U.S. Pat. No. 4,367,755 is of sufficient thickness and mechanical strength to withstand tensile and shear forces upon removal of the electrode from the patient, or movement and adjustment of the electrode upon the patient. These properties deleteriously influence the mass transport capability of the electrode, and hence, both the breatheability of the electrode and the ability of the electrode to transport water away from the underlying skin surface.
Moreover, the absorption of sweat into the conductive, adhesive pad serves to plug the channels or micropores within the pad, which further reduces both the breatheability of the electrode and the ability of the electrode to transport water away from the underlying skin surface.
Finally, it will be appreciated by one skilled in the field of mass transfer that the diffusion of air from the environment into the pad, and the diffusion, from the pad, of air containing moisture from the skin surface is impeded by the tortuosity of the diffusion path. As noted by Charles N. Satterfield in “Heterogeneous Catalysis in Practice” (McGraw-Hill, Inc., 1980, p. 336), “the length of the tortuous diffusion path in real pores is greater than the distance along a straight line in the mean direction of diffusion. Moreover, the channels through which diffusion occurs are of irregular shape and of varying cross section; constructions offer resistances that are not offset by the enlargements.”
Satterfield further articulates that “if the gas density is low or if the pores are quite small, or both, the molecules collide with the pore wall much more frequently than with each other . . . . The gas flux is reduced by the wall ‘resistance’”. Hence, the prior art electrodes, in which relatively thick pads contact the skin surface, and in which the pad materials have characteristically small pores and high tortuosity factors, are generally incapable of providing sustained mechanical and electrical performance over long-term use of two weeks or more.
It will be appreciated that U.S. Pat. No. 4,367,755, like U.S. Pat. No. 4,300,575, teaches away from a conductive interface layer having macroscopic exposed surface area, because such a configuration compromises the efficacy of the adhesive coating, reduces the electrical contact area, and reduces the mechanical strength to withstand tensile and shear forces upon removal of the electrode from the patient, which is the main inventive thrust of U.S. Pat. No. 4,300,575.
Furthermore, the stagnant conditions underneath a cast, coupled with the extremely long duration in which the cast encompasses or covers the skin surface (typically 3–10 weeks), render the above-described electrodes even more inadequate for transporting sweat accumulated on the surface of the skin, through the various conductive and adhesive pads, to the atmosphere. These stagnant conditions accelerate the process in which the above-described electrodes swell due to water absorption, lose their form, and move out of the proper position for transmitting the electrical signals.
The need for surface electrodes suitable for long-term electrical function is long-standing. Such surface electrodes are needed for receiving various kinds of electrical signals transmitted from within the body, and for delivering an electrical signal to the body, as in the case of electrical stimulation of muscle and/or bone tissue covered by a cast.
It would be highly advantageous, therefore, to have a surface electrode having sustained mechanical, physical and electrical performance over long-term storage and use, so as to enable transcutaneous electrical communication in a safe, comfortable, reliable, and effective manner, even under difficult topical and ambient conditions.